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General Aspects

3.1. General Aspects

Electrical energy is generated in large hydroelectric, thermal and nuclear power stations.

These stations are mostly situated away from the load centres. Therefore an extensive power supply network is necessary between the generating plants and consumers loads.

By transmission and distribution of electric power is meant its conveyance from the central station where it is generated to the places where it is demanded by the consumers (like pumping stations, residential and commercial buildings, mills, factories etc.)

  • The maximum voltage in advanced countries is 33 kV while that in India is 11 kV.
    • The amount of power that has to be transmitted through transmission lines is very large and if this power is transmitted at 11 kV (or 33 kV) the line current and the power loss would be very large. Therefore this voltage is stepped up to a higher value by using step-up transformers located in sub-stations.
    • The transmission voltages in India are 400 kV, 220 kV and 132 kV.
      • The high voltage transmission lines transmit electrical power from the generating stations to main receiving end sub-stations. At these stations the voltage is stepped down to a lower value of66 kV or 33 kV. 1

        Fig. 9. A typical power supply network.

        • The secondary transmission system forms the link between the main receiving and sub-stations and secondary sub-stations. At the secondary sub-stations the voltage is stepped down to 33 kV or 11 kV and the power is fed into the primary distribution system.
          • The 33 kV or 11 kV distribution lines (usually known as feeders) emanate from the secondary sub-stations and terminate in distribution sub-stations. The distribution sub-stations consist of step-down transformers and are located at convenient places in the area in which the power is to be supplied. Sometimes these distribution sub-stations consist of pole mounted transformers located on the road side. These transformers step down the voltage to 400 V.
            • The 400 V distribution lines (usually known as distributors) are laid along the roads and service connections to consumers are tapped off from the distributors.
            • All transmission and distribution systems are 3-phase systems.
              • The transmission lines and feeders are 3-phase 3 wire circuits.
                • The distributors are 3-phase 4 wire circuits because a neutral wire is necessary to supply the single phase loads of domestic and commercial consumers.

        The transmission network is commonly known as ‘grid’.

        Fig. 9 shows a typical power supply network.